Noise-reduction systems and methods using photonic bandgap crystals

ABSTRACT

Active electronic circuits are immersed in photonic bandgap crystals (PBC&#39;s) that form part of transmission lines for propagation of output signals of the electronic circuits. The output signals of the electronic circuits are accompanied by noise signals that result from spontaneous emission in emission frequency bands which are associated with the active electronic circuits. The PBC&#39;s are configured to have photonic bandgaps that include at least a portion of the emission frequency bands. Because the active electronic circuits are immersed in the photonic bandgap crystal, the launch of at least a portion of the noise signals into the transmission line is thereby inhibited. Consequently, the output signal and less than all of the noise signals are propagated along the transmission line, i.e., the noise content of the circuit output is reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to low-noise electronic systems.

2. Description of the Related Art

Electrical noise is a ubiquitous phenomenon in electronic devices and ittypically sets a lower bound on the sensitivity of electronic systems.Electrical noise generally includes thermal noise and shot noise.Thermal noise is generated by random thermal motion of charged particlesand is associated with thermodynamic energy exchanges that maintainthermal equilibrium between a circuit and its surroundings. In contrast,shot noise is generated by the random passage of discrete currentcarriers across barriers or discontinuities (e.g., semiconductorjunctions).

Two other noise components originate in low-frequency conductancefluctuations within electrical devices. The first component exhibits aLorentzian frequency dependence in its power spectral density. It isreferred to as G-R noise because it originates from fluctuations in thenumber of free electrons in device conduction bands that are caused bygeneration and recombination processes between the bands and interactingtraps. The second component exhibits a 1/f.sup.α (0.4<α<1.2) powerspectral density. Although its generation is not well understood, amultitude of mechanisms appear to generate it including superposition ofG-R spectra with different characteristic times and weights.

The performance of active electronic circuits is degraded by thepresence of noise. In a first exemplary degradation, the noise figure oflow-noise amplifiers (LNA) is increased. Receiver noise figure issimilarly increased because it is primarily determined by the noisefigure of the receiver's LNA. Excess noise in LNA's typically manifestsitself in device signal fluctuations (e.g., current fluctuations in thegate and drain of field-effect transistors). Oscillator phase noise isincreased in a second exemplary degradation. Phase noise in the outputsignal of oscillators generally results from upconversion of lowfrequency noise. In a third exemplary degradation, phase noise is addedto the output of clock circuits which lowers the performance of systemsassociated with the clock. For example, phase noise in sampling clocksdecreases the dynamic range of analog-to-digital converters.

Conventional methods for reducing noise signals in electronic circuitshave generally included the steps of, a) designing electronic devicestructures with reduced surface area, b) employing materials andprocesses with favorable carrier generation/recombination parameters andc) selecting active devices that exhibit low excess noisecharacteristics.

Regardless of the nature of an active device, excess noise is physicallyassociated with statistical processes (e.g., carrier generation andrecombination) at various device locations (e.g., surface/passivationinterfaces and bulk interfaces such as junctions and heterojunctions).Whatever the specific model adopted to interpret excess noise frequencydependence, conductance fluctuations (which produce measurable voltagefluctuations) are caused by spontaneous emission of atomic carriers. Incontrast to stimulated emission which is induced by the presence ofradiant energy of like frequency and wavelength, spontaneous emission ina quantum mechanical system is radiation that is emitted when theinternal system energy spontaneously drops from an excited state to alower state without regard to the simultaneous presence of similarradiation.

A reference on spontaneous emission (Yablonovitch, Eli, "InhibitedSpontaneous Emission in Solid-State Physics and Electronics", TheAmerican Physical Society, Vol. 58, No. 20, May 18, 1987, pp.2059-2062), points out that it is neither feasible nor desirable toeliminate spontaneous emission entirely if a function of a semiconductorstructure (e.g., a laser or a solar cell) is the emission or absorptionof light. Rather, the goal in those cases is to restrict spontaneousemission to those electromagnetic modes that are absolutely necessary.

This reference observes that periodic spatial modulation (e.g., indistributed-feedback lasers and interference coatings for wave optics)opens up a forbidden gap in the electromagnetic dispersion relation. Forexample, three-dimensional spatial periodicity of λ/2 in the refractiveindex can result in a forbidden gap in the electromagnetic spectrum nearthe wavelength λ. If the electromagnetic band gap overlaps an electronicband edge, then electron-hole radiative recombination (hence,spontaneous emission) will be severely inhibited.

The reference concludes that inhibited spontaneous emission is a realpossibility in semiconductor lasers but requires further materialsdevelopment before the benefits are fully realized. With respect toheterojunction bipolar transistors, the reference teaches minimizing oftransistor electron-hole recombination with consequent enhancement oftransistor current gain. Because of conflicting requirements (e.g., highbase doping to obtain low series resistance and high speed operation),the reference concludes that this application of inhibited spontaneousradiation would be limited to transistors of moderate base doping.

A second reference (Sigalas, M. M., et al., "Metallic Photonic Band-gapMaterials", The American Physical Society, Vol. 52, No. 16, October1995, pp. 11744-11751) compares metallic photonic band-gap structures todielectric photonic bandgap crystals (PBC's). It calculates transmissionand absorption characteristics of electromagnetic waves fortwo-dimensional and three-dimensional periodic structures. Intwo-dimensional metallic structures, it was determined that propagatingmodes of s-polarized waves are interrupted by band gaps (behaviorsimilar to that of dielectric PBC's) while p-polarized waves exhibit acutoff frequency below which propagating modes are severely attenuated.Three-dimensional metallic structures with isolated metallic scattererswere found to behave similar to dielectric PBC's but continuous networksof metallic scatterers were found to have no propagating modes below acutoff frequency for both s-polarized and p-polarized waves.

A third reference (Sievenpiper, M. M., et al., "3D Wire Mesh PhotonicCrystals", The American Physical Society, Vol. 76, No. 14, April 1996,pp. 2480-2483) describes three dimensional wire mesh structures having ageometry similar to covalently bonded diamond. Similar to dielectricPBC's, the frequency and wave vector dispersion show forbidden bands atfrequencies ν_(o) corresponding to the lattice spacing. In addition,they have a forbidden band extending from zero frequency to ˜1/2 ν_(o).

As defined in a fourth reference (Brown, E. R., et al., "RadiationProperties of a Planar Antenna on a Photonic-Crystal Substrate", Journalof the Optical Society of America, Vol. 10, No. 2, February 1993, pp.404-407), a photonic bandgap crystal (PBC) is a periodic structure thatexhibits a forbidden band of frequencies (i.e., a photonic bandgap) inits electromagnetic dispersion.

This latter reference introduces PBC's as a substrate material forplanar antennas and describes an experimental "bow tie" microstripantenna that was fabricated by adhering copper tape to surfaces of aPBC. The PBC had a bandgap between 13 and 16 GHz and was fabricated bydrilling holes in an epoxy-based dielectric having a dielectric constantof ˜13. The radiation performance of this experimental antenna wascompared with that of a conventional antenna that was fabricated with asolid substrate of the same dielectric material. Measured radiationpatterns of the second antenna indicated that it radiated primarily intoits substrate with a lesser, useful radiation into the air. In contrast,measured radiation patterns of the first antenna indicated that itsradiation was predominately confined as useful radiation into the air.In a summary of the experimental antenna's performance, it was statedthat the PBC substrate expels radiation by Bragg scattering and,consequently, radiation is neither trapped in the substrate norreflected back at such a phase as to lower the resistance of theantenna's driving point.

Although these references describe various PBC structures and teach theuse of a PBC in expelling radiation from a substrate, they fail toprovide any guidance to noise-reduction in active circuits (i.e.,circuits having components which perform dynamic functions such asamplification, oscillation and signal modification).

SUMMARY OF THE INVENTION

The present invention is directed to noise-reduction structures andmethods that have wide-ranging applications. These goals are achieved byusing photonic bandgap crystals (PBC's) to inhibit electromagnetic-modepropagation within forbidden regions of the PBC's and immersing activecircuits in the PBC's to inhibit launching of noise signals in theforbidden regions.

Output signals at an output port of an active electronic circuit aretypically accompanied by noise signals that result from spontaneousemission of electromagnetic radiation in an emission frequency band thatis associated with the active electronic circuit. Accordingly, noisereduction is realized by launching the output signal into a transmissionline for propagation and by coupling at least the output port portion ofthe active electronic circuit to a photonic bandgap crystal which has aphotonic bandgap that includes at least a portion of the emissionfrequency band.

Consequently, the launch into the transmission line of at least aportion of the noise signals is inhibited. Thus, the output signal andless than all of the noises signals are propagated along thetransmission line, i.e., the signal-to-noise ratio is improved.

Essentially, the coupling step immerses the active electronic circuit inthe photonic bandgap crystal. In a first system embodiment, theimmersion is achieved by configuring a substrate of a planartransmission line to form a photonic bandgap crystal and coupling theoutput port to a signal line of this transmission line. In a secondsystem embodiment, the immersion is achieved by establishing a PBC in awaveguide and coupling the output port to the waveguide.

In practicing the teachings of the invention, transmissioncharacteristics of various PBC's (e.g., dielectric and metallictwo-dimensional and three-dimensional PBC's) can be selectively matchedto correspond to the emission frequency bands of different activeelectronic circuits.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a low-noise active electronic system of thepresent invention;

FIG. 2 is a plan view of another low-noise active electronic system;

FIGS. 3A-3D are graphs which illustrate transmission characteristics ofdifferent photonic bandgap crystals in the systems of FIGS. 1 and 2;

FIGS. 4A-4C are block diagrams of different active electronic circuitsin the system of FIG. 1;

FIGS. 5A and 5B are plan and elevation views of another low-noise activeelectronic system; and

FIGS. 6A-6C are flow charts which illustrate noise-reduction processesin the low-noise active electronic systems of FIGS. 1, 4, 5A and 5B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Transmission lines exemplified by microstrip lines, strip lines, slotlines, and coplanar lines are typically referred to as planartransmission lines because their characteristics are determined bydimensions in a single plane. In contrast with coaxial and waveguidetransmission lines, the structures of planar transmission lines lendthemselves to photolithographic fabrication techniques and facilitatetheir connection with and integration into electronic circuits.

FIG. 1 illustrates a low-noise active electronic system 20 in which anactive circuit 22 is associated with a planar transmission line in theform of a microstrip transmission line 24. In FIG. 1, the microstriptransmission line 24 is broken away in one corner to show that itincludes a substrate 26, a conductive ground plane 28 and a conductivesignal line 30. The ground plane and the signal line are carried onopposed sides of the substrate.

The substrate 26 includes spatially-periodic structures 32 that aredefined by a dielectric member 33 (e.g., a ceramic such as alumina or apolymer such as fluorocarbon plastic). In the embodiment of FIG. 1, thestructures 32 are holes which are orthogonally arranged with the groundplane 28. The dielectric member 33 and its spatially-periodic structures32 form a dielectric photonic bandgap crystal (PBC), i.e., the substrate26 is a dielectric PBC.

The microstrip transmission line 24 conducts output signals of theactive circuit 22 away from its output port 34 to a transmission lineoutput 35. Some active electronic circuits may also have a signal inputport 36 for reception of input signals from a transmission line input37. Preferably, interconnections within the active electronic circuit 22are arranged to also form microstrip structures with the substrate 26and the ground plane 28.

FIG. 2 illustrates another low-noise active electronic system 40. FIG. 2is similar to FIG. 1 with like elements indicated by like referencenumbers. In the low-noise system 40, however, the planar transmissionline 24 of the low-noise system 20 is replaced by a planar transmissionline 44. This latter transmission line is similar to the transmissionline 24 but has a substrate 46 in which the spatially-periodicstructures 32 of the dielectric member 33 of FIG. 1 have been filledwith metal (e.g., copper) to form spatially-periodic metallic structuresin the form of posts 48. Accordingly, the dielectric member 33 and itsspatially-periodic posts form a metallic photonic bandgap crystal (PBC),i.e., the substrate 46 is a metallic PBC.

In operation of the low-noise systems 20 and 40, the active circuit 22generates output signals which are launched onto the microstriptransmission lines 24 and 44 and propagated to the transmission lineoutput 35. In some active circuits (e.g., oscillators) the outputsignals are generated without need for any input. In other activecircuits (e.g., low-noise amplifiers) the output signals are generatedin response to input signals which are conducted from the transmissionline input 37 to the input port 36.

The output signals of the active circuit 22 are accompanied by noisesignals which result from spontaneous emission of electromagneticradiation in an emission frequency band that is associated with theactive circuit 22. In conventional electronic systems, these noisesignals would be launched onto the transmission lines 24 and 44 with theoutput signals and propagated to the transmission line output 35.Because the noise signals appear with the output signals at the output35, they degrade the system's performance.

In contrast, the planar transmission lines 24 and 44 of the inventionare configured so that they inhibit the launching and subsequentpropagation of at least a portion of the noise signals. In particular,the substrates 26 and 46 of the planar transmission lines are configuredas PBC's which have forbidden regions in their transmissioncharacteristics. Typically, the transmission-forbidden regions can bepositioned to substantially cover the emission frequency band ofspontaneous emission that is associated with the active circuit 22 whileavoiding the operating frequency of the active circuit.

As signals of the active circuit 22 travel along the microstriptransmission lines 24 and 44, a small portion of their electromagneticfields extend through the air above the transmission lines but the majorportion of these fields is contained within the substrates 26 and 46.Accordingly, the functional processes of the active circuit 22 aresubstantially immersed within the PBC's that are formed by thesesubstrates. Because of this immersion, the launching of the noisesignals into the transmission lines 24 and 44 is inhibited within thePBC forbidden regions.

The forbidden regions are configured to avoid the output signal regionsof the active circuit 22. Accordingly, the transmission of the activecircuit's output signals is not affected and their electromagnetic modespropagate along the planar transmission lines 24 and 44 to thetransmission line output port 35. In comparison to conventionalelectronic systems, therefore, the signal-to-noise ratio is improved atthe output 35.

FIGS. 3A-3C illustrate transmission plots of exemplary dielectric andmetallic PBC's. A dielectric PBC is one having spatially-periodicdielectric structures (e.g., spatially-periodic holes). A metallic PBCis one having spatially-periodic metallic structures (e.g.,spatially-periodic wires or posts).

A variety of different transmission plots can be obtained withcombinations of different electromagnetic modes and different dielectricand metallic PBC structures. PBC transmission plots also vary dependingon whether the periodic structure of the PBC is two-dimensional (i.e.,periodic only in two dimensions) or three-dimensional (i.e., periodic inthree dimensions). The plots of FIGS. 3A-3C are only exemplary of thosewhich have been documented in numerous references (e.g., the referencesrecited above in the background section).

In particular, the graph 60 of FIG. 3A shows a rejection band 62 in atransmission plot 64 which is characteristic of both dielectric andmetallic PBC's. The graph 70 of FIG. 3B shows a transmission plot 72which has a cutoff frequency 74 below which transmission is severelyattenuated. This high-pass shape is characteristic of many metallicPBC's.

By introducing defects (i.e., discontinuities) in the periodic structureof both dielectric and metallic PBC's, a passband can be introducedwithin a rejection band. This is exemplified by the transmission plot 82of the graph 80 of FIG. 3C. This plot is similar to the plot 64 of FIG.3A but has a passband 84 within the rejection band 62. As shown in thegraph 90 of FIG. 3D, three-dimensional metallic PBC's can be configuredto have a transmission plot 92 which exhibits both a cutoff frequency 94and a higher-frequency rejection band 96. In addition, the introductionof defects in the spatially-periodic metallic structure can cause apassband 98 to appear below the cutoff frequency 94.

The spacing of spatially-periodic structures to obtain transmissionplots exemplified by those of FIGS. 3A-3D has been well documented inthe PBC art. For example, the frequency of the rejection band 96 in FIG.3D represents a wavelength which substantially corresponds to theperiodic spacing while the cutoff frequency 94 represents a wavelengthwhich substantially corresponds to one half of the periodic spacing.

The output signals of active electronic circuits are typicallyaccompanied by noise signals that result from spontaneous emission ofelectromagnetic radiation in emission frequency bands that areassociated with the active electronic circuit. These active electroniccircuits can be immersed in transmission lines whose PBC substrates areselected so that their transmission characteristics (as exemplified inFIGS. 3A-3D) have forbidden regions which correspond to the circuits'emission frequency bands.

FIGS. 4A-4C illustrate examples of the active electronic circuit 22 ofFIGS. 1 and 2. A low-noise amplifier (LNA) 100 is included in a receiver102 of FIG. 4A for initial amplification of an input signal from theinput 37. The output port 34 of FIGS. 1 and 2 is located at theamplifier's output. Subsequently, the amplified signal is downconvertedin a mixer 104 for further amplification in an intermediate-frequencyamplifier 106. A downconversion signal is supplied to the mixer 104 by alocal oscillator (LO) 108. A bandpass filter (BPF) 110 precedes themixer 104 to reduce spurious input signals while a lowpass filter (LPF)112 follows the mixer to reduce spurious mixing signals.

As stated above, the LNA 100 primarily determines the noise figure ofthe receiver 102. A substantial portion of the excess noise of LNA'sappears as modulation sidebands about the amplifying frequency. That is,excess noise results from spontaneous emission of electromagneticradiation in an emission frequency band and the emission frequency bandassociated with the LNA 100 is the region surrounding the amplifiedsignal. An appropriate corresponding PBC transmission characteristic forthe LNA 100 may therefore be the transmission plot 82 shown in FIG. 3C.

Because low-frequency noise is also upconverted to appear in the LNA'soutput, another emission frequency band associated with the LNA 100 isthe region below the amplified signal. Accordingly, another appropriatecorresponding PBC transmission characteristic for the LNA 100 may be amodified version of the transmission plot 92 of FIG. 3D. In this case,it would be modified by removing the passband 98 and the defect in thespatially-periodic metallic structure which generated it.

FIG. 4B illustrates an oscillator 120 having an amplifier 122 and afeedback path 124 from the amplifier's output port 34 to its input port36. A substantial portion of the phase noise of oscillators isdetermined by upconversion of low-frequency noise. Therefore, anemission frequency band associated with the oscillator 120 lies belowthe oscillator output frequency. An appropriate corresponding PBCtransmission characteristic for the oscillator 120 may therefore be thetransmission plot 72 of FIG. 3B.

In FIG. 4C, an analog-to-digital converter (ADC) 130 converts analogsignals at an analog input 132 to digital signals at a digital output134. This conversion is accomplished with the timing supplied by asampling clock 136. Noise at the clock's output port 34 degrades thedynamic range of the ADC. Because the emission frequency bandsassociated with the clock 136 are similar to those of the oscillator 120of FIG. 4B, appropriate corresponding PBC transmission characteristicsmay also be that of FIG. 3B.

The teachings of the invention can be extended to transmission linesother than planar transmission lines. For example, FIGS. 5A and 5B aresimilar to FIG. 1 (with like elements indicated by like referencenumbers) except that the output port 34 has been adapted to couplesignals into a waveguide transmission line 142. In an active electronicsystem 140, the signal line 30 of the planar transmission line 24 hasbeen extended as a probe 143 which couples to electromagneticpropagation modes in a waveguide 144. Metal posts 146 are arranged in alattice to form a PBC 148. The output of the waveguide transmission lineis at a waveguide end which carries an attachment flange 149.

Although the illustrative PBC 148 has a two-dimensionalspatially-periodic metallic structure, the waveguide 144 canalternatively be configured with three-dimensional structures (e.g., athree-dimensional wire mesh). The PBC 148 would typically be configuredto have a transmission characteristic (e.g., one of the transmissionplots of FIGS. 3A-3D) which is selected to conform to the noise emissionfrequency band of its active electronic circuit 22.

FIGS. 6A-6C illustrate noise-reduction processes in the low-noise activeelectronic systems of FIGS. 1, 2, 5A and 5B. In particular, FIG. 6Ashows a process 160 which has a first process step 162 in which anoutput signal is generated at an output port of an active electroniccircuit. Unfortunately, this output signal is accompanied by theunwanted contribution of noise signals. That is, a process 163 is not anintended process but is, instead, an unwanted process that results fromspontaneous emission in an emission frequency band that is associatedwith the active electronic circuit. The broken connection line 164indicates that step 163 is an involuntary step.

In step 166, the output signal is launched into a transmission line forpropagation away from the electronic circuit's output port. In step 167,at least the output port portion of the active electronic circuit iscoupled to a photonic bandgap crystal which has a photonic bandgap thatincludes at least a portion of the emission frequency band. Because atleast a portion of the active electronic circuit is thereby immersed inthe photonic bandgap crystal, the launch of at least a portion of thenoise signals into the transmission line is inhibited. Therefore, theoutput signal and less than all of the noise signals are propagatedalong the transmission line in process step 168.

The coupling process of step 167 immerses the electronic circuit in aphotonic bandgap crystal. In detail, this action is initiated in flowchart 170 by providing a substrate-based transmission line (e.g., aplanar transmission line) in step 172. In step 174, a plurality ofspatially-periodic structures are formed in a substrate of thetransmission line to generate a photonic bandgap (PBG) that includes atleast a portion of the emission frequency band (recited in step 167 ofFIG. 6A). Finally, the electronic circuit is immersed in the photonicbandgap crystal by coupling its output port in step 176 to a signal lineof the transmission line. Preferably, a substantial portion of theelectronic circuit is also carried by other signal lines of thetransmission line.

Another immersion process is detailed in the flow chart 180 of FIG. 6C.This process is initiated in step 182 by providing a waveguidetransmission line. In step 184, a plurality of spatially-periodicmetallic members are positioned within the waveguide to generate aphotonic bandgap (PBG) that includes at least a portion of the emissionfrequency band. Finally, the electronic circuit is immersed in thephotonic bandgap crystal by coupling its output port in step 186 to thewaveguide.

Although the teachings of the invention have been illustrated withreference to two-dimensional PBC's, they may be practiced also withthree-dimensional PBC's. Although the active electronic circuit 22 ofFIGS. 5A and 5B has been shown to be coupled into the waveguidetransmission line 142 via a planar transmission line 24, otherembodiments of the invention can be formed in which the active circuitand the waveguide transmission line are directly coupled.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. Such variations and alternate embodimentsare contemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

I claim:
 1. A low-noise active electronic system, comprising:an activeelectronic circuit having an output port and generating an output signalat said output port which is accompanied by noise signals that aregenerated by spontaneous emission of electromagnetic radiation in anemission frequency band associated with said active electronic circuit;a planar transmission line coupled to said output port to receive andpropagate said output signal wherein said planar transmission lineincludes:a) a substrate with said active electronic circuit positionedover said substrate and supported by said substrate; b) a signal linecarried by said substrate and coupled to said output port; and c) aground plane carried by said substrate and spaced from said signal line;and a photonic bandgap crystal coupled to said output port, saidphotonic bandgap crystal formed by spatially-periodic structures in saidsubstrate that are configured to have a photonic bandgap which includesat least a portion of said emission frequency band so that launching ofat least a portion of said noise signals onto said transmission line isinhibited, said transmission line thereby propagating said output signaland less than all of said noise signals.
 2. The low-noise activeelectronic system of claim 1, wherein said active electronic circuit iscarried on said substrate.
 3. The low-noise active electronic system ofclaim 1, wherein said substrate is comprised of a ceramic.
 4. Thelow-noise active electronic system of claim 1, wherein said substrate iscomprised of a fluorocarbon polymer.
 5. The low-noise active electronicsystem of claim 1, wherein said spatially-periodic structures are holesformed by said substrate.
 6. The low-noise active electronic system ofclaim 1, wherein said spatially-periodic structures are metal posts. 7.The low-noise active electronic system of claim 1, wherein saidspatially-periodic structures have two-dimensional periodicity.
 8. Thelow-noise active electronic system of claim 1, wherein saidspatially-periodic structures have three-dimensional periodicity.
 9. Thelow-noise active electronic system of claim 1, wherein said transmissionline is a microstrip transmission line.
 10. The low-noise activeelectronic system of claim 1, wherein said active electronic circuitincludes a low-noise amplifier coupled to said output port.
 11. Thelow-noise active electronic system of claim 1, wherein said activeelectronic circuit includes an oscillator coupled to said output port.12. The low-noise active electronic system of claim 1, wherein saidactive electronic circuit includes a clock coupled to said output port.13. A low-noise active electronic system, comprising:an activeelectronic circuit having an output port and generating an output signalat said output port which is accompanied by noise signals that aregenerated by spontaneous emission of electromagnetic radiation in anemission frequency band associated with said active electronic circuit;a transmission line coupled to said output port to receive and propagatesaid output signal; and a photonic bandgap crystal coupled to saidoutput port and configured to have a photonic bandgap which includes atleast a portion of said emission frequency band so that launching of atleast a portion of said noise signals onto said transmission line isinhibited, said transmission line thereby propagating said output signaland less than all of said noise signals; wherein at least a portion ofsaid transmission line is a waveguide and said photonic bandgap crystalcomprises a plurality of spatially-periodic metallic members positionedwithin said waveguide.
 14. The low-noise active electronic system ofclaim 13, wherein said metallic members are metallic posts.
 15. Thelow-noise active electronic system of claim 13, wherein saidspatially-periodic metallic members have two-dimensional periodicity.16. The low-noise active electronic system of claim 13, wherein saidspatially-periodic metallic members have three-dimensional periodicity.17. A low-noise active electronic system, comprising:an activeelectronic circuit having an output port and generating an output signalat said output port which is accompanied by noise signals that aregenerated by spontaneous emission of electromagnetic radiation in anemission frequency band associated with said active electronic circuit;a transmission line coupled to said output port to receive and propagatesaid output signal; and a photonic bandgap crystal coupled to saidoutput port and configured to have a photonic bandgap which includes atleast a portion of said emission frequency band so that launching of atleast a portion of said noise signals onto said transmission line isinhibited, said transmission line thereby propagating said output signaland less than all of said noise signals; wherein: said transmission lineincludes first and second coupled transmission line portions: said firsttransmission line portion has:a) a substrate; b) a signal line carriedby said substrate and coupled to said output port; and c) a ground planecarried by said substrate and spaced from said signal line; and saidsecond transmission line portion is a waveguide; and said photonicbandgap crystal includes a first photonic bandgap crystal portion formedby spatially-periodic structures in said substrate, and a secondphotonic bandgap crystal portion formed by spatially-periodic metallicmembers in said waveguide.
 18. The low-noise active electronic system ofclaim 17, wherein said signal line extends into said waveguide to couplesaid first and second transmission line portions.
 19. A method ofreducing noise signals in an output signal of an active electroniccircuit, comprising the steps of:generating an output signal at anoutput port of an active electronic circuit wherein said output signalis accompanied by noise signals that are generated by spontaneousemission of electromagnetic radiation in an emission frequency bandassociated with said active electronic circuit; launching said outputsignal into a transmission line for propagation away from said outputport; and coupling at least the output port portion of said activeelectronic circuit to a photonic bandgap crystal which has a photonicbandgap that includes at least a portion of said emission frequency bandto thereby inhibit the launch of at least a portion of said noisesignals into said transmission line, said output signal and less thanall of said noise signals thereby propagated through said transmissionline; wherein said coupling step includes the steps of: providing awaveguide as said transmission line; positioning a plurality ofspatially-periodic metallic members within said waveguide to form saidphotonic bandgap crystal; and coupling said output port to saidwaveguide.
 20. A method of reducing noise signals in an output signal ofan active electronic circuit wherein said output signal is accompaniedby noise signals that result from spontaneous emission ofelectromagnetic radiation in an emission frequency band associated withsaid active electronic circuit, said method comprising the stepsof;positioning said active electronic circuit over a planar transmissionline so that it is supported by said planar transmission line; launchingsaid output signal into said planar transmission line for propagationaway from said output port; defining a plurality of spatially-periodicstructures in a substrate of said planar transmission line to therebyform a photonic bandgap crystal with a photonic bandgap that includes atleast a portion of said emission frequency band; and coupling at leastthe output port portion of said active electronic circuit to saidphotonic bandgap crystal to thereby inhibit the launch of at least aportion of said noise signals into said transmission line, said outputsignal and less than all of said noise signals thereby propagatedthrough said transmission line.
 21. A method of reducing noise signalsin an output signal of an active electronic circuit wherein said outputsignal is accompanied by noise signals that result from spontaneousemission of electromagnetic radiation in an emission frequency bandassociated with said active electronic circuit, said method comprisingthe steps of;launching said output signal into a transmission line forpropagation away from said output port; and coupling at least the outputport portion of said active electronic circuit to a photonic bandgapcrystal which has a photonic bandgap that includes at least a portion ofsaid emission frequency band to thereby inhibit the launch of at least aportion of said noise signals into said transmission line, said outputsignal and less than all of said noise signals thereby propagatedthrough said transmission line; wherein said coupling step includes thesteps of: providing a waveguide as said transmission line; positioning aplurality of spatially-periodic metallic members within said waveguideto form said photonic bandgap crystal; and coupling said output port tosaid waveguide.